Catadioptric imaging systems for the deep ultraviolet spectral region (about 0.15 to 0.30 μm wavelength) are known. U.S. Pat. No. 5,031,976 to Shafer and U.S. Pat. No. 5,488,229 to Elliott and Shafer disclose two such systems. These systems employ lens elements made from only a single refractive material, namely fused silica, since it is practically the only material that combines good transmission of deep UV light with desirable physical properties. For example, fluoride glasses (based on CaF2, LiF, etc.), while transmissive of deep UV light, are generally considered too soft, making lens formation difficult. Thus, fluoride glass materials are normally avoided whenever possible.
In the above-noted '976 Shafer patent, an optical system is disclosed, which is based on the Schupmann achromatic lens principle producing an achromatic virtual image, and which combines it with a reflective relay to produce a real image. The system, reproduced here as FIG. 7, includes an aberration corrector group of lenses 101 for providing correction of image aberrations and chromatic variation image aberrations, a focusing lens 103 receiving light from the group 101 for producing an intermediate image 105, a field lens 107 of the same material as the other lenses placed at the intermediate image 105, a thick lens 109 with a plane mirror back coating 111 whose power and position is selected to correct the primary longitudinal color of the system in conjunction with the focusing lens 103, and a spherical mirror 113 located between the intermediate image and the thick lens 109 for producing a final image 115. Most of the focusing power of the system is due to the spherical mirror 113. It has a small central hole 117 near the intermediate image 105 to allow light from the intermediate image 105 to pass therethrough to the thick lens 109. The mirror coating 111 on the back of the thick lens 109 also has a small central hole 119 to allow light focused by the spherical mirror 113 to pass through to the final image 115. While primary longitudinal (axial) color is corrected by the thick lens 109, the Offner-type field lens 107 placed at the intermediate image 105 has a positive power to correct secondary longitudinal color. Placing the filed lens slightly to one side of the intermediate image 105 corrects tertiary longitudinal color. Thus, axial chromatic aberrations are completely corrected over a broad spectral range. The system incidentally also corrects for narrow band lateral color, but fails to provide complete correction of residual (secondary and higher order) lateral color over a broad UV spectrum.
The above-noted '229 patent to Elliott and Shafer provides a modified version of the optical system of the '976 patent, which has been optimized for use in 0.193 μm wavelength high power excimer laser applications, such as ablation of a surface 121′, as seen in FIG. 8. This system has the aberration corrector group 101, focusing lens 103′, intermediate focus 105′, field lens 107′, thick lens 109′, mirror surfaces 111′ and 113′ with small central openings 117′ and 119′ therein and a final focus 115′ of the prior '976 patent, but here the field lens 107′ has been repositioned so that the inter-mediate image or focus 105′ lies outside of the field lens 107′ to avoid thermal damage from the high power densities produced by focusing the exciter laser light. Further, both mirror surfaces 111′ and 113′ are formed on lens elements 108′ and 109′. The combination of all light passes through both lens elements 108′ and 109′ provides the same primary longitudinal color correction of the single thick lens 109 in FIG. 7, but with a reduction in total glass thickness. Since even fused silica begins to have absorption problems at the very short 0.193 μm wavelength, the thickness reduction is advantageous at this wavelength for high power levels. Though the excimer laser source used for this optical system has a relatively narrow spectral line width, the dispersion of silica near the 0.193 μm wavelength is great enough that some color correction still needs to be provided. Both prior systems have a numerical aperture of about 0.6.
Longitudinal chromatic aberration (axial color) is an axial shift in the focus position with wavelength. The prior system seen in FIG. 7 completely corrects for primary, secondary and tertiary axial color over a broad wavelength band in the near and deep ultraviolet (0.2 μm to 0.4 μm). Lateral color is a change in magnification or image size with wavelength, and is not related to axial color. The prior system of FIG. 7 completely corrects for primary lateral color, but not for residual lateral color. This is the limiting aberration in the system when a broad spectral range is covered.
An object of the invention is to provide a catadioptric imaging system with correction of image aberrations, chromatic variation of image aberrations, longitudinal (axial) color and lateral color, including residual (secondary and higher order) lateral color correction over a broad spectral range in the near and deep ultraviolet spectral band (0.2 to 0.4 μm).
In addition to color correction, it is also desired to provide a UV imaging system useful as a microscope objective or as microlithography optics with a large numerical aperture for the final image and with a field of view of at least 0.5 mm. The system is preferably telecentric.